1. Field of the Invention
This invention relates to annularly ribbed tube and to a method and apparatus for its production.
2. Background of the Invention
It has become conventional to use corrugated tube, possibly double-walled corrugated tube, or annularly ribbed tube for thermoplastic, semi-rigid tubing frequently used as drainage conduit for underground use, or for carrying electrical cables. A corrugated or double-walled corrugated tube has some advantages and annular ribbed tube has other advantages. For example, corrugated tube, whether or not double-walled, tends to be of lighter construction than ribbed tube. It may be made by blow molding techniques which do not require such high extrusion pressures as do the injection molding techniques which are required for ribbed tube. On the other hand, corrugated tube is not as sturdy as ribbed tube and, when it is double-walled, special provisions must be made to allow for equalization of pressure within the closed corrugated envelopes between the tube walls so that these have no tendency to collapse. Various other considerations, possible specific to the intended use, apply to the choice of ribbed or corrugated tube.
The techniques necessary for the production of double-walled corrugated tube are quite well known in this field. For example, extrusion nozzles having coaxial extrusion orifices for extruding annual parisons to form each of the walls are known from U.S. Pat. No. 4,226,580 (Lupke) issued October 1980, 30 U.S. Pat. No. 4,305,703 (Lupke) issued December 1981, U.S. Pat. No. 4,500,284 (Lupke) issued February 1985, U.S. Pat. No. 4,510,013 (Lupke) issued April 1985, and U.S. Pat. No. 4,534,923 (Lupke) issued August 1985.
The Patents quoted above are merely exemplary of a large number of patents in this field.
Ribbed tube differs from corrugated tube by its greater rigidity which is, inter alia influenced by the longitudinal spacing of annular or helical ribs which upstand relatively sharply from an outer tube surface, at least in comparison to the smooth waves of corrugated tube. The tube wall, including the ribs, is generally relatively dense and the tube is, therefore, heavy.
The thickness of the ribs in the longitudinal direction of the tube may be small in comparison with the distance of tube between each rib. Tube between each rib may be profiled with patterning in the form of embossed or otherwise formed annular ridges or other patterns. Such patterns are not to be confused with the sharply upstanding ribs to which the invention relates. The ribs typically, although not necessarily, upstand from the surface of the tube by at least a height equal to the tube wall radial thickness. Possibly, the rib height is as much as five times or more the radial tube thickness.
As a result of the difference in structure between ribbed tube and corrugated tube, the molding techniques in forming ribbed tube are more akin to injection molding techniques than to the blow molding used for the outer skin of double walled corrugated pipe due to the extrusion pressure necessary to force plastic extrudate into the grooves to mold the ribs of the pipe. Thus, although, filled corrugations have been contemplated in the past (see German Offenlegunschrift 2,042,031), it has not been thought possible to provide ribbed pipe except as a single continuous injection extrusion.
German Offenlegunschrift 2,042,031 discloses a blow molding technique in which an outer corrugated wall is produced in a conventional manner. Downstream of this, again in a somewhat conventional manner an inner smooth wall is imposed within the corrugated wall. The German document discloses foaming the inner wall so that the extrudate expands to fill the corrugations in addition to forming the inner wall. Ribbed tube could not be produced in this manner.
Although ribbed tube as previously known is somewhat heavy and extravagant in raw material, its advantages in rigidity and strength are considerable. While considerable effort has been applied to the problem of proper penetration of the rib forming grooves by the extrudate, the problem of undue usage of plastic extrudate appears to have been regarded as insoluble.
The present invention has addressed the problem of providing a ribbed tube which is (1) lighter than previously thought possible, (2) less extravagant in materials, (3) may incorporate recycled material and (4) is of comparable quality to conventional ribbed tube.